As mobile devices have been increasingly developed, and the demand for such mobile devices has increased, the demand for secondary batteries has also sharply increased as an energy source for the mobile devices. Among such secondary batteries is a lithium secondary battery having high energy density and high discharge voltage, on which much research has been carried out and which is now commercialized and widely used.
Depending upon the shape of a battery case, a secondary battery may be classified as a cylindrical battery having an electrode assembly mounted in a cylindrical metal container, a prismatic battery having an electrode assembly mounted in a prismatic metal container, or a pouch-shaped battery having an electrode assembly mounted in a pouch-shaped case formed of an aluminum laminate sheet.
The electrode assembly mounted in the battery case is a power generating element, having a cathode/separator/anode stack structure, which can be charged and discharged. The electrode assembly may be classified as a jelly roll type electrode assembly configured in a structure in which a long sheet type cathode and a long sheet type anode, to which active materials are applied, are wound while a separator is disposed between the cathode and the anode or a stacked type electrode assembly configured in a structure in which pluralities of cathodes and anodes having a predetermined size are sequentially stacked while separators are disposed respectively between the cathodes and the anodes.
Secondary batteries may be exposed to various environments according to use status and conditions. It is necessary to prevent explosion of the secondary batteries for the sake of safety of users. Generally, the batteries may explode due to high temperature and high pressure in the batteries caused by an abnormal operational status, such as an internal short circuit, charging exceeding allowable current and voltage, exposure to high temperature, impact due to falling, of the batteries. For these reasons, each of the batteries is provided with a high pressure solving device for solving high pressure of the battery, which is a direct cause for battery explosion, although the shapes of the batteries are different from one another.
In order to solve high pressure, for example, a cylindrical battery has a safety plate of a specific structure, which is mounted to a cap assembly, a prismatic battery has a safety groove formed at a cap or a case of the battery, and a pouch-shaped battery is configured such that sutured portions (sealed portions) of laminate sheets are separated from each other without use of an additional safety groove.
In a general prismatic secondary battery, closed type or a partially open type safety groove is formed at an aluminum battery case such that the safety groove can be cut out.
For example, a prismatic secondary battery has a partially open type safety groove formed at the side of a battery case.
A safety groove 30 of FIG. 1 is formed at a corner of the side of a case 20 of a prismatic secondary battery in the shape of a small contour. The safety groove is of a partially open type. That is, the safety groove is located at a region of the case exhibiting relatively high stress such that the curved safety groove can be ruptured when the internal pressure of the battery is excessively increased.
The safety groove having the structure as described above has an advantage in that the safety groove can relatively sensitively respond to high pressure generated in the battery; however, the safety groove has a disadvantage in that it is difficult to correctly set an intended critical pressure value intended during designing of the battery.
That is, as previously described, high stress is applied to the corner region of the side of the case, with the result that the safety groove may be easily ruptured even at low pressure. Above all, when the thickness of the battery case is small, the safety groove more sensitively responds to high pressure, with the result that unintended rupture of the safety groove is caused.
For this reason, it is necessary to decrease the size and depth of the safety groove formed at the region exhibiting high stress. When the size and depth of the safety groove are too small, however, the safety groove may not be easily ruptured.
Also, the shape of the safety groove is expected to be a very important factor in achieving reliable operation of the battery under abnormal conditions of the battery.
Therefore, there is a high necessity for developing a prismatic battery that is capable of quickly discharging gas from the battery through uniform rupture of a safety grove when the internal pressure of the battery is increased in synthetic consideration of thickness of a battery case, the position of the safety groove based on stress, and the shape, length, and depth of the safety groove.